A solenoid valve has been widely used for controlling flow amount and pressure of fluid. In the solenoid valve, it has been known so-called a linear solenoid valve using a linear solenoid which controls flow amount and pressure of fluid by be in proportion to electric current flowed to a coil, with controlling a magnetic attractive force of a plunger which is controlled by an amount of electric current flowed to the coil.
A structural example of such the conventional solenoid valve is shown in FIG. 2.
A solenoid valve 9 shown in FIG. 2 comprises a coil 20 which generates a magnetic field by electric current, a center post 40b wherein a magnetic circuit is formed by the magnetic field generated by the coil 20, a plunger 30 which is magnetically attracted to the center post 40b, a valve 59 integrally provided with the plunger 30 having a rod 50 and a valve element 55, a valve seat 60 having a through hole 61 contacted with the valve element 55 so as to be opened and closed, a side ring 70 provided so as to enclose a peripheral of the plunger 30, a valve body 80 wherein a bobbin 81 and a cylindrical portion 82 are integrally provided, and a case 90 which is a cylindrical shape having bottom, a spring 10 which defines a position of the plunger 30 by pulling the plunger to a bottom face of the case 90.
Also, the solenoid valve 9 comprises a bearing 11 which supports the plunger 30 and a bearing 12 which supports the rod 50. A first through hole 83 and a second through hole 84 which communicate with an internal portion and an external portion of the valve body 80 respectively are formed at both ends of the valve seat 60 of a cylindrical portion 82 of the valve body 80.
By the constitution, the solenoid valve 9 controls flow amount or pressure of fluid flew outs from an output port 85 by opening and closing the valve element 55 of the valve 59 to the through hole 61 of the valve seat 60 as being the first through hole 83 is an input port for controlled fluid, an edge opening 85 of the cylindrical portion 82 is an output port, the second through hole 84 is a drain port.
In the solenoid valve of this kind, a position of the valve 59 is defined by depending balance of a fluid pressure (e.g., oil pressure) received by the valve 59, an attractive force of which the center post 40b pulls the plunger 30, and a spring force of the spring 10. A pressure control of the controlled fluid which is output from the output port 85 is adjusted by controlling the position of the valve 59. However, when the valve 59 is activated, the controlled fluid such as oil and the like flows into an internal side of the solenoid 95 from a space between the valve 59 and the bearing 12 and a space between the plunger 30 and the bearing 11, there are some cases that an effect is given to a control position of the valve by that a force other than the above mentioned force acts to the valve 59 due to effects of fluctuation of the controlled fluid (oil) and air ratio in the solenoid or a temperature.
Therefore, in the conventional solenoid valve, although it is not shown in FIG. 2, there is a case that fluid and air which are flown into an inside of the solenoid (95) are inlet and outlet by providing a through hole at a bottom face (upper face) of the case (90) so as to activate as a drain port.
Also, for example, a solenoid valve shown in Japanese Patent Publication No. 2005-299919 (Patent Document 1), although it is not shown in FIG. 2, it is suggested that a groove 73 is formed at a contact face side with a bottom face of a case 90 of a flange portion 72 of a side ring 70, an air through hole which communicate with an inner portion 95 of the solenoid valve 95 and an outer portion of the solenoid valve is formed by the groove 73 so that the above mentioned effect is reduced by the air through hole which is as for so called a breathing bore (herein after there is a case called as a breathing bore 73).
In the meantime, some contamination is included in the controlled fluid such as oil and the like, if they are flown into the solenoid valve, there is a case they are adhered to the inside of the solenoid.
In particular, a solenoid valve used for an automatic transmission includes high density of a magnetic contamination in the oil by abrasion of the valve and the like, also, it is necessary to adjust a pressure for a line pressure controlling by activating the solenoid valve continuously, therefore, more large amount of oil which includes contamination flows into or flows out to the solenoid valve. Also, as mentioned above, in the solenoid valve wherein the drain port or the breathing bore 73 are provided at an upper portion of the case, the controlled fluid such as oil and the like flows into the solenoid positively by providing such bore.
Then, the contamination flown into the solenoid adheres in the solenoid due to receiving an effect of a magnetic field formed in the solenoid. In a constitution of the above mentioned solenoid, the contamination easily accumulates particularly at a position between the center post and the plunger which exchanges a magnetic flux, namely at a position where a magnetic attractive forces of the center post and the plunger are acting. Note that, although the contamination adheres to both plunger and the center post which form a magnetic circuit, the contamination hardly adheres to the plunger due to frequently moving thereof, the contamination tends to accumulate easily at the center post since it is not moving.
And if the contamination is accumulated at the position exchanging the magnetic flux of the center post and peripheral position thereof, there is a case to provide an adverse effect to a control property (controlling pressure property) of the solenoid valve, due to changing a magnetic permeability of the magnetic circuit and gap lengths of an axial direction and a radial direction of an attractive portion become ununiformly.
For responding to such problem, for example, a solenoid valve disclosed in a gazette of Japanese Patent No. 3666246 (Patent Document 2), it has been proposed to eliminate adhesion of the contamination by coating nonmagnetic material to a plunger. However, as mentioned above, since magnetic contamination accumulates and build-up mainly at a center post, the above mentioned disadvantageous matter cannot be eliminated sufficiently, even coating the nonmagnetic material at the plunger only. Also, by a constitution wherein the nonmagnetic material is coated at the plunger surface, there is a case that the coating is removed when assembling due to insufficient bonding power of the nonmagnetic material, there is a high possibility to prevent the adhesion of the magnetic contamination to the plunger cannot be prevented. Thus, it has been desired a method for preventing the magnetic contamination more effectively.    Patent Document 1: Gazette of Japanese Patent Laid Open No. 2005-299919    Patent Document 2: Gazette of Japanese Patent No. 3666246